Hybrid vehicles generally use motor generators at slow engine speeds, at which the motor generators have better torque characteristics than internal combustion engines, and use internal combustion engines at moderate to fast speeds, at which the engines have better torque characteristics. This improves fuel efficiency, as the engine is not used at low speeds.
A battery is used to drive the motor generator or store electricity generated by the motor generator. When an SOC (state of charge) of the battery is in an appropriate range, the battery can supply sufficient power to smoothly drive the motor generator and store the electricity generated by the motor generator. If the battery is used when the SOC is not in the appropriate range, it is difficult to control the motor generator to a desired state, and the life of the battery is drastically reduced.